This invention relates in general to electronic brake control systems and in particular to anti-lock brake control systems.
An Anti-lock Brake System (ABS) is often included as standard equipment on new vehicles. When actuated, the ABS is operative to modulate the pressure applied to some or all of the vehicle wheel brakes. A typical ABS includes a plurality of solenoid valves mounted within a control valve body and connected to the vehicle hydraulic brake system. The valve body also includes an accumulator for the temporary storage of brake fluid during an anti-lock braking cycle.
A separate hydraulic source, such as a motor driven pump, is usually included in the ABS for reapplying hydraulic pressure to the controlled wheels during an ABS braking cycle. The pump is typically disposed within the control valve body with the pump motor mounted upon the exterior of the control valve body. The pump motor is usually a direct current brushed motor that operates from the vehicle power supply. Typically, the motor runs continuously during an ABS braking cycle. However, the motor speed is controlled by exciting the motor with a constant frequency Pulse Width Modulated (PWM) voltage that is generated by a semiconductor based motor drive circuit. The PWM duty cycle is reduced toward the end of an ABS brake cycle to slow the motor and thereby reduce motor noise.
An ABS further includes a microprocessor disposed within an Electronic Control Module (ECU) that is mounted upon the control valve body. The microprocessor is electrically connected to a semiconductor based pump motor drive circuit, which is also disposed within the ECU, a plurality of solenoid coils associated with the solenoid valves and wheel speed sensors for monitoring the speed and deceleration of the controlled wheels. The assembled control valve body, motor and control module form a compact unit which is often referred to as an ABS control valve.
During vehicle operation, the microprocessor in the ABS control module continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to initiate an ABS braking cycle. During the ABS braking cycle, the microprocessor actuates the pump motor and selectively operates the solenoid valves in the control valve to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the vehicle operator.
The microprocessor includes a memory portion which stores an ABS control algorithm. The ABS control algorithm comprises a set of instructions for the microprocessor which control the operation of the ABS. Typically, the instructions include a set of operational checks which are run during vehicle start up to assure that the ABS is functional. The control algorithm also includes subroutines for monitoring the vehicle operation to detect a potential lock-up of the controlled wheel brakes and for the actual operation of the ABS during an anti-lock braking cycle. The control algorithm may also include subroutines that selectively operate the solenoid valves and pump to provide Traction Control (TC) and Vehicle Stability Control (VSC) to the vehicle.
As described above, the ABS includes a semiconductor based motor drive circuit. The motor drive circuit is designed to operate under a worse case scenario. Accordingly, during an extended ABS braking cycle, such as may be encountered when a vehicle is stopped upon a very low mu surface, the extended operation of the motor may cause excessive heating of the motor drive circuit. Additionally, because the motor drive circuit is disposed within the ECU, the other components contained therein also are exposed to the heat generated by the motor drive circuit. Therefore, it would be desirable to reduce the heat generated by motor drive circuit during an extended ABS braking cycle.